More robust handling of type variables in mcomp

[trefis]

When comparing two types, Ctype.mcomp asserts that if neither of them is a variable, then they can't both be expanded to a variable.
The first commit adds an example of a code to the testsuite showing that this assertion doesn't hold.

The second commit removes the assertion and handles variables in the exact same way as before expansion of the types.
MPR#6158 and the related commit seem to confirm that this is the correct fix.

The handling of variables pre-expansion is redundant, but I left it in place to serve as a short-circuit since expansion might be costly.
I however doubt that it will be triggered often in practice, as "external" calls to mcomp are always preceeded by calls to reify.

[garrigue]

Well spotted.
The fix is trivial, so please go forward and merge it.